refactor(virtio): Simplify read_config

`io::Write` has higher overhead and unnecessary error handling. Slices can be copied without it.

Signed-off-by: Kornel <kornel@cloudflare.com>

src/vmm/src/devices/virtio/balloon/device.rs

@@ -5,7 +5,7 @@ use std::io::Write;
 use std::sync::atomic::AtomicU32;
 use std::sync::Arc;
 use std::time::Duration;
-use std::{cmp, fmt};
+use std::fmt;
 
 use log::error;
 use serde::Serialize;
@@ -594,19 +594,11 @@ impl VirtioDevice for Balloon {
     }
 
     fn read_config(&self, offset: u64, mut data: &mut [u8]) {
-        let config_space_bytes = self.config_space.as_slice();
-        let config_len = config_space_bytes.len() as u64;
-        if offset >= config_len {
+        if let Some(config_space_bytes) = self.config_space.as_slice().get(u64_to_usize(offset)..) {
+            let len = config_space_bytes.len().min(data.len());
+            data[..len].copy_from_slice(&config_space_bytes[..len]);
+        } else {
             error!("Failed to read config space");
-            return;
-        }
-
-        if let Some(end) = offset.checked_add(data.len() as u64) {
-            // This write can't fail, offset and end are checked against config_len.
-            data.write_all(
-                &config_space_bytes[u64_to_usize(offset)..u64_to_usize(cmp::min(end, config_len))],
-            )
-            .unwrap();
         }
     }
 

src/vmm/src/devices/virtio/block/vhost_user/device.rs

@@ -4,7 +4,6 @@
 // Portions Copyright 2019 Intel Corporation. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 
-use std::cmp;
 use std::io::Write;
 use std::sync::atomic::AtomicU32;
 use std::sync::Arc;
@@ -323,18 +322,12 @@ impl<T: VhostUserHandleBackend + Send + 'static> VirtioDevice for VhostUserBlock
     }
 
     fn read_config(&self, offset: u64, mut data: &mut [u8]) {
-        let config_len = self.config_space.len() as u64;
-        if offset >= config_len {
+        if let Some(config_space_bytes) = self.config_space.as_slice().get(u64_to_usize(offset)..) {
+            let len = config_space_bytes.len().min(data.len());
+            data[..len].copy_from_slice(&config_space_bytes[..len]);
+        } else {
             error!("Failed to read config space");
             self.metrics.cfg_fails.inc();
-            return;
-        }
-        if let Some(end) = offset.checked_add(data.len() as u64) {
-            // This write can't fail, offset and end are checked against config_len.
-            data.write_all(
-                &self.config_space[u64_to_usize(offset)..u64_to_usize(cmp::min(end, config_len))],
-            )
-            .unwrap();
         }
     }
 

src/vmm/src/devices/virtio/net/device.rs

@@ -7,7 +7,6 @@
 
 #[cfg(not(test))]
 use std::io::Read;
-use std::io::Write;
 use std::net::Ipv4Addr;
 use std::sync::atomic::AtomicU32;
 use std::sync::{Arc, Mutex};
@@ -822,19 +821,12 @@ impl VirtioDevice for Net {
     }
 
     fn read_config(&self, offset: u64, mut data: &mut [u8]) {
-        let config_space_bytes = self.config_space.as_slice();
-        let config_len = config_space_bytes.len() as u64;
-        if offset >= config_len {
+        if let Some(config_space_bytes) = self.config_space.as_slice().get(u64_to_usize(offset)..) {
+            let len = config_space_bytes.len().min(data.len());
+            data[..len].copy_from_slice(&config_space_bytes[..len]);
+        } else {
             error!("Failed to read config space");
             self.metrics.cfg_fails.inc();
-            return;
-        }
-        if let Some(end) = offset.checked_add(data.len() as u64) {
-            // This write can't fail, offset and end are checked against config_len.
-            data.write_all(
-                &config_space_bytes[u64_to_usize(offset)..u64_to_usize(cmp::min(end, config_len))],
-            )
-            .unwrap();
         }
     }